The multiple hopping transmission system using OFDM (Orthogonal Frequency Division Multiplexing) is known. This multiple hopping transmission system is the radio network that enables the user to communicate with the mobile terminal in a wider range since not only the mobile terminals can communicate directly with each other but also the mobile terminals can communicate mutually via other mobile terminal. In this radio network, the user can not only communicate with the base station in the cell to which the user's mobile terminal belongs but also access the base station out of the cell via other mobile terminal.
FIG. 1 shows a radio relay system as a premise assumed to communicate by using a plurality of radio relay devices. In FIG. 1, the base station BS applies an error correction coding to transmitted data, and generates a signal C1 and a signal C2. Here, the signal C1 and the signal C2 have a different redundancy respectively. For example, C1=systematic bit S and C2=parity bit are assumed.
A signal C1(S) and a signal C2(P) are transmitted to a mobile station MS (mobile terminal) through repeaters RS (radio relay units) provided to different propagation paths, i.e., a repeater RS1 and a repeater RS2. In this case, slots through which the signal C1(S) and the signal C2(P) are transmitted respectively are separate slots, and the repeater RS1 reproduces the signal C1(S) and repeats it and the repeater RS2 reproduces the signal C2(P) and repeats it. The mobile station MS executes an error correction decoding by using the signal C1(S) and the signal C2(P).
In this manner, when two repeaters RS1, RS2 pass on the signal from the base station BS to the mobile station MS respectively, these repeaters RS1, RS2 pass on a different system (a systematic bit S and a parity bit P) respectively to achieve the diversity effect.
In this case, the error correction system such as LDPC (Low Density Parity Check Codes), turbo code, or the like is executed as the error correction system. In the turbo code, two types of information of the systematic bit S and the parity bit P having different quality are generated as the error correction bit. The systematic bit S are more important than the parity bit P to the error correction.
As shown in FIG. 1, first the base station BS transmits the systematic bit S and the parity bit P to the repeater RS1 and the repeater RS2 by using two frames (#1, #2) respectively. Then, the repeater RS1 passes on the signal in a next frame #3, and the repeater RS2 passes on the signal in a frame #4 after next. The mobile station MS receives both signals from the repeater RS1 and the repeater RS2, and demodulates both received signals by an error correction decoding process.
In the process in the repeater RS in the prior art, the signal is transferred while applying a hard decision (the received signal has a value in a range of −∞ to +∞, but the signal is converted into −1 if the value is negative and converted into 1 if the value is positive) to the received signal. That is, a hard decision signal is transferred irrespective of the presence or absence of the error in the received signal. The reason why the signal is transmitted as it is even though such signal contains the error is that, because there is a possibility that the error can be corrected by the error correction decoding at the end receiving station, the final receiving performance may be improved and therefore the signal had better be transmitted (see Patent Literature 1, for example).    Patent Literature 1: JP-A-2001-189971